Rechargeable batteries, especially high-energy-density lithium-ion (“Li-ion”) batteries, have been used in a variety of applications, such as power trains of electric vehicles (EVs) and hybrid electric vehicles (HEVs), consumer electronics, such as electric appliances, and aerospace and military applications due to their high energy density, high output power density, rapid charge capacity, rapid discharge capacity, and high open circuit voltage. These energy storage devices (ESDs) are required to operate at high charge and discharge rates to support loads for different applications. Even small form power batteries can provide very large currents. For example, A123's 26650 power batteries (ANR26650M1A) can provide a maximum 70 A continuous discharge rate and a maximum 10 A charge rate for a single cell.
However, most rechargeable Li-ion batteries are temperature sensitive. Li-ion batteries typically have an operating temperature range of −20-60° C., with an optimal operating temperature range of 25-40° C. and module-to-module temperature differences of less than 5° C. Storage or use of batteries at temperatures over 50° C. significantly lowers their discharge efficiency and longevity. For example, the capacity of Li-ion batteries has been shown to decrease 99+% after storage at 60° C. The power capacity of cells that experience 800 charge-discharge cycles-cells without significant drop in capacity under typical conditions dropped to less than 40% after 600 cycles at 50° C. and to less than 30% at 55° C.
Heat generation is more severe for cylindrical batteries compared to prismatic batteries because cylinders have a low surface-area-to-volume ratio. Research showed that during discharge, 54% of the generated heat is from ohmic resistance, 30% is from electrochemical reactions, and 16% is from activation polarization. It has been observed that heat generation can easily raise the battery skin temperature by 15° C. at a relatively low discharge rate of 1.7 C (5A). When such a temperature raise occurs, the battery may have to operate at a temperature close to its upper limit, resulting in significant performance degradation. For example, A123's ANR26650M1 A batteries, if discharged at the highest current of 70 A (28 C), generate a significant amount of heat (˜30 W/cell). This amount of heat may not be problematic if the battery is directly exposed to an ambient temperature of 25° C. However, it can be a significant problem if multiple batteries are tightly packed in a battery pack and experience fast cycling between charge and discharge. In this case, the batteries have to be down-rated to 30 A (12 C) discharge for the battery pack instead of 70 A (28 C) for the single cell to avoid decreased capacity, shortened battery life, safety issues, or even catastrophic accidents
In order to achieve high power capacity at reasonable voltages, hundreds of the types of batteries described above are typically packed closely in a finite space. Maintaining an appropriate temperature during operation is especially difficult with current battery chemistries and architectural designs when operating at rapid charge and discharge rates.
Battery thermal management methods can be classified according to their cooling method and media with common types being air-cooled, liquid-cooled, heat pipe, phase change material (PCM)-based thermal storage, thermoelectric, and cold plate, as shown in Table 1 and FIGS. 1a-c.
TABLE 1Summary of thermal management strategiesThermal Management StrategiesForced AirLiquidHeat pipePCMThermoelectricCold plateEase of useEasyDifficultModerateEasyModerateModerateIntegrationEasyDifficultModerateEasyModerateModerateEfficiencyLowHighHighHighLowMediumTemperature dropSmallLargeLargeLargeMediumMediumTemperature distributionUnevenEvenModerateEvenModerateModerateMaintenanceEasyDifficultModerateEasyDifficultModerateLife>20 years3-5 years>20 years>20 years1-3 years>20 yearsInitial costLowHighHighModerateHighHighAnnual costLowHighModerateLowHighModerate
Thermoelectric cooling and cold plate systems are least preferred due to a low coefficient of performance and poor contacting on the heat transfer interface, respectively. Forced air and liquid cooling approaches are active cooling approaches, which utilize external energy to achieve fast heat transfer. Typical air cooling systems have been widely employed in automobiles because of their simplicity and availability; however, they usually have decreased energy density due to the space required for air path channels. Air-cooling also requires loosely packed arrays with decreased energy density to allow sufficient air flow.
Liquid-cooled systems allow larger temperature drops and uniform temperature distributions relative to air-cooled systems, which can allow for very compact battery cooling design. These cooling systems have several common configurations including discrete tubing around each module, a cooling jacket around the modules, direct contact with (i.e. submersion in) a dielectric fluid, and placing the modules on a liquid cooled plate (e.g., heat sink). Both direct- and indirect-contact approaches, however, are required to be leak-free and short-circuit free. Direct contact approaches can achieve acceptable cooling performance; however, dielectric fluids are required to avoid short circuits. As a result, liquid-cooled systems are characterized by their short life and high cost.
Indirect contact approaches are typically more reliable than direct contact approaches, but their performance is hindered due to poor contacting between the batteries and cooling tubing/plates. Strategies that utilize cold plates and water jackets are also indirect-contact designs and exhibit the same limitations.
Phase change materials (PCM)-based cooling structures have been explored for cooling batteries and battery arrays. PCMs utilize the latent heat capacity of phase changes (i.e. solid-liquid or solid-solid phase changes) to capture and store heat. These structures store heat during fast battery charge and discharge and reduce the peak temperatures and release heat back to the environment slowly during off-peak operation (e.g. charge). Conventional PCM thermal management approaches do not necessarily need fast heat exchange with the environment as long as the heat exchange is sufficient enough to release enough heat to prevent overheating during off-peak operations. As a passive cooling approach, conventional PCM systems can be simple, reliable, low-weight, and compact. However, because of the slow rate at which they release heat to the surrounding environment, conventional PCM systems are considered thermal (enthalpy) storage systems rather than an effective external steady state cooling systems. For this reason, PCMs are not suitable for battery arrays with frequent, rapid charge and discharge cycles or for battery arrays used in environments where the ambient temperature is close to or above the melting point of PCMs.
PCMs also exhibit poor thermal conductivity when in the solid phase. Typical PCMs have thermal conductivities ranging from 0.2 to 0.5 W/m-K. The highest thermal conductivities of PCMs are 2.1 W/m-K for liquids and 0.53 W/m-K for solids. These low thermal conductivities may be sufficient for type 18650 cells because the intercell gaps are small; however, they are generally not sufficient for large cell arrays where the gaps are much larger. In this case, PCM close to the batteries may melt and the PCM far from the batteries may not be utilized. This can result in severe battery overheating and poor utilization of the PCM.
There exists a need for improved thermal management systems that effectively remove or disperse heat from high energy density batteries, such as rechargeable lithium-ion batteries.